Just found out my prescriptions have been paid by my ex's insurance company without my knowledge of this. Now?

I was divorced 4 yrs ago and knew I would have not health coverage. I was taken off his health plan and knew I would be paying out of pocket. I signed up for a plan with the pharmacy to get generics at a discount and soon after was put on medicaid. After medicaid ran out I was sure I was back on the pharmacy plan that was for people who didn't have coverage. I just found out that the pharmacy has been billing the ex's insurance company. I have been in the pharmacy and on the phone with pharmacy and his insurance and none of this is being resolved. The ex has informed me that there were some dr. bills fro 09 that were pd by his insurance also, while I was on medicaid. I am getting the run around from his ins. and pharmacy. Why would ins even accept payment? What do I do?

My ex has threatened to speak with an attorney if I don't fix this mistake. I have spent countless hrs on the phone and it seems I am getting nowhere. His insurance company has given me 3 different dates as to when my coverage ended. Yet they continued paying for prescriptions when I didn't have coverage through them. Do I need to go to where I got medicaid to alert them to this also?